In a typical cellular radio communication system (wireless communication system), an area is divided geographically into a number of cell sites, each defined by a radio frequency (RF) radiation pattern from a respective base transceiver station (BTS) antenna. The base station antennae in the cells are in turn coupled to a radio node controller (RNC) or a base station controller (BSC), which is then coupled to a telecommunications switch or gateway, such as a mobile switching center (MSC) and/or a packet data serving node (PDSN) for instance. The switch or gateway may then be coupled with a transport network, such as the PSTN or a packet-switched network (e.g., the Internet).
Most cells in a wireless network are usually further divided geographically into a number of sectors, each defined respectively by radiation patterns from directional antenna components of the respective BTS, or by respective BTS antennae. These sectors (which can be visualized ideally as pie pieces) can be referred to as “physical sectors,” “cell sectors,” or just “sectors,” since they correspond to physical areas of a cell site. At any given instant, an access terminal (such as a cellular telephone, pager, or appropriately equipped portable computer, for instance) in a wireless network will typically be positioned in a given physical sector and will be able to communicate with the transport network via the BTS serving that physical sector.
According to the wireless cellular protocol IS-856, an access terminal regularly monitors the signal strength of pilot signals that the access terminal receives from the sectors in its vicinity. Signal strength is typically measured as a carrier-to-interference (C/I) ratio, or a signal-to-interference-plus-noise ratio (SINR), for example. The access terminal then selects as a serving sector the sector whose pilot signal has the highest signal strength, and the access terminal uses that signal strength as a basis to select a data rate at which to request the sector to transmit data to the access terminal. In particular, IS-856 defines a fixed mapping between SINR (or C/I) and “Data Rate Control” (DRC) codes, with each DRC code corresponding to a given range of SINR (or C/I) values and defining a particular data rate. Applying that mapping, the access terminal selects a DRC code and sends a DRC request over an air interface control channel to the access network, identifying (i) the DRC code and (ii) the selected sector, by a “DRC_cover” value.
IS-856 operates in a time division multiplex (TDM) manner by dividing air interface communications in each sector into timeslots, and with timeslots synchronized on all sectors in an access network's coverage area. Advantageously, bearer data communications in each timeslot can then be transmitted from the access network to the access terminal at the full sector power without interfering with transmissions to other access terminals in that sector. Consequently, the chance of successful data transmission to the access terminal is greatly increased, and the overall throughput of the sector tends to increase.
Each DRC code that an access terminal sends to the access network defines a requested data rate by corresponding with a modulation scheme and a maximum number of interlaced timeslots that the access network will use when it attempts to transmit a radio link layer data packet to the access terminal. In particular, the higher the DRC, the fewer the timeslots, and thus in theory the quicker the data transmission will occur to the access terminal. This correspondence logically follows from the fact that a higher DRC corresponds with a higher SINR (or C/I), which means that air interface conditions are better and should therefore be able to support a higher data-rate modulation scheme, and thus quicker successful transmission to the access terminal.
For a given DRC corresponding with a particular number of timeslots, the access network repeatedly attempts transmission of the packet to the access terminal in sequential (interlaced) timeslots, adding more error correction coding in each successive timeslot and/or transmitting various portions of the packet with various error correction coding in each successive timeslot, with the goal that the access terminal will ultimately receive enough data to constitute or facilitate uncovering of the packet as a whole. For instance, if the packet payload comprises the elements ABCD, the access network may transmit in the first timeslot the full payload ABCD plus some error correction coding. If that transmission is insufficient, the access network may then transmit in the next timeslot a portion of the payload, such as ABC, plus some additional error correction coding. And if that transmission is still insufficient, the access network may then transmit in the next timeslot another portion of the payload, such as BDC, plus more substantial error correction coding. This process would continue until the packet transmission is successful or until the number of timeslots is exhausted (in which case the transmission would have failed).
During this packet transmission process, for each timeslot that does not result in the access terminal having successfully received or uncovered the complete packet payload, the access terminal transmits a negative acknowledgement (NACK) to the access network, to prompt the access network to keep trying if additional timeslots remain. On the other hand, once the access terminal has received the complete packet, the access terminal transmits a positive acknowledgement (ACK) to the access network, which tells the access network that transmission of the packet was successful. If successful receipt of a given packet occurs before the allocated number of timeslots have passed, the access network then proceeds to the next packet, saving timeslot resources by not having to re-transmit the given packet or a portion of the given packet again.
According to IS-856, if an access terminal has been receiving transmissions on a given sector and discovers that another sector is exhibiting a stronger signal-to-noise ratio (i.e., SINR or C/I), the access terminal will ask the access network to begin serving the access terminal on the new sector instead. In particular, once the access terminal finishes receiving any packet currently being transmitted on the current sector, the access terminal would begin the handoff process by sending a series of null DRC messages on its current sector, to notify the access network that the access terminal is about to hand off to another sector, so that the access network will hold any packets destined for the access terminal until the handoff is complete. The access terminal would then begin sending DRCs on the new sector, to cause the access network to begin transmitting packets to the access terminal on the new sector.